What are acid-fast bacillus (AFB) tests?
Acid-fast bacillus (AFB) is a type of bacteria that causes tuberculosis and certain other infections. Tuberculosis, commonly known as TB, is a serious bacterial infection that mainly affects the lungs. It can also affect other parts of the body, including the brain, spine, and kidneys. TB is spread from person to person through coughing or sneezing.
TB can be latent or active. If you have latent TB, you’ll have TB bacteria in your body but won’t feel sick and can’t spread the disease to others. If you have active TB, you’ll have symptoms of the disease and could spread the infection to others.
AFB tests are usually ordered for people with symptoms of active TB. The tests look for the presence of AFB bacteria in your sputum. Sputum is a thick mucus that is coughed up from the lungs. It is different from spit or saliva.
There are two main types of AFB tests:
- AFB smear. In this test, your sample is “smeared” on a glass slide and looked at under a microscope. It can provide results in 1–2 days. These results can show a possible or likely infection, but can’t provide a definite diagnosis.
- AFB culture. In this test, your sample is taken to a lab and put in a special environment to encourage the growth of bacteria. An AFB culture can positively confirm a diagnosis of TB or other infection. But it takes 6–8 weeks to grow enough bacteria to detect an infection.
Other names: AFB smear and culture, TB culture and sensitivity, mycobacteria smear and culture
What are they used for?
AFB tests are most often used to diagnose an active tuberculosis (TB) infection. They may also be used to help diagnose other types of AFB infections. These include:
- Leprosy, a once feared, but rare and easily treatable disease that affects the nerves, eyes, and skin. Skin often becomes red and flaky, with loss of feeling.
- An infection similar to TB that mostly affects people with HIV/AIDS and others with weakened immune systems.
AFB tests may also be used for people who have already been diagnosed with TB. The tests can show if the treatment is working, and whether the infection can still be spread to others.